|Publication number||US7246530 B2|
|Application number||US 10/544,671|
|Publication date||Jul 24, 2007|
|Filing date||Jan 26, 2004|
|Priority date||Feb 10, 2003|
|Also published as||EP1592949A1, EP1592949B1, US20060096389, WO2004070323A1|
|Publication number||10544671, 544671, PCT/2004/97, PCT/SE/2004/000097, PCT/SE/2004/00097, PCT/SE/4/000097, PCT/SE/4/00097, PCT/SE2004/000097, PCT/SE2004/00097, PCT/SE2004000097, PCT/SE200400097, PCT/SE4/000097, PCT/SE4/00097, PCT/SE4000097, PCT/SE400097, US 7246530 B2, US 7246530B2, US-B2-7246530, US7246530 B2, US7246530B2|
|Inventors||Hans Hallstadius, Göran Ohlsson, Sven Gustafson, Bengt Jonasson, Gert-Inge Bertinsson|
|Original Assignee||Gambro Lundia Ab|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Classifications (12), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is an national phase application based on POT/SE2004/1000097, filed Jan. 26, 2004, which claims the priority of Sweden Application No. 0300342-3, filed Feb. 10, 2003, and the benefit of U.S. Provisional Application No. 60/523,723, filed Nov. 19, 2003, the content of each of which is incorporated herein by reference in its entirety.
The present invention relates generally to measurement of a fluid flow. More particularly the invention relates to a flow sensor according to the preamble of claim 1, an ultra-filtration measuring unit according to the preamble of claim 8 and a method for measuring a flow rate component according to the preamble of claim 10.
There are many technical areas in which an accurate fluid flow measurement is required. Numerous representative applications can, for instance be found in the field of medical technology. However, some kind of flow measurement is normally required in most instances where a fluid/liquid is to be transported from one point to another. Fluid flow measurements may also be used to determine the velocity of a craft traveling in water or a similar liquid. Naturally, mechanical flow sensors may here be employed. Nevertheless, when a high accuracy is required, electromagnetic flow measurement sensors are generally preferable. This type of sensors are based on a well-known technique where a magnetic field is applied to interact with electrically charged elements, such as ions, in the flowing fluid to produce a resulting electric field. Thus, according to Faraday's law, the magnitude of the induced electric field constitutes a measure of the flow rate of the fluid.
The applicant develops and manufactures medical equipment, for instance renal products in the form of dialysis apparatuses, which utilize such an electromagnetic flow measurement technique. Commonly, flow sensors with platinum electrodes have been employed to deliver accurate and reliable values of the registered fluid flows. Platinum, and platinum black in particular, namely accomplishes a good electrical contact between the electrodes and the dialysis liquid, so that the electrode dimensions can be held comparatively small, and consequently give rise to a linear magnetic field pattern in the fluid conduit cross section.
In recent years, a so-called glucose-charging practice has been introduced wherein glucose is added to the dialysis liquid in order to better imitate the composition of the patient's own blood, and thereby i.a. avoid certain metabolic risks for the patient. However, glucose also produces undesired effects due to its electrochemical activity with platinum. During operation of a dialysis apparatus with a glucose-charged dialysis liquid, a catalytic reaction causes glucose to be oxidized on the flow sensors' platinum surfaces. This decreases the reliability of the flow measurements. Namely, the oxidation may result in a varying DC-level (DC=Direct Current) at the sensor electrodes, which in turn renders it difficult to determine the contribution to the registered electric field caused by the electromagnetic interaction between the applied magnetic field and the charged elements in the flowing liquid (i.e. here ions in the dialysis liquid).
The U.S. Patent Application No. 2002/0050175 describes a magnetic flow sensor and method, wherein an undesired drift of the electrode voltages is compensated for, either by interconnecting the sensing electrodes or by connecting them to a common potential, such as ground. Also in this case, a voltage indicative of the flow rate is measured by means of at least two electrodes. A high-impedance voltage-measurement circuit is used to register a voltage between the electrodes. Thus, during the measurement, the electrodes are in an open circuit state, and may therefore be electrically influenced by electrode polarization and other measurement error-inducing factors that develop relatively slowly. In order to avoid such effects, the electrodes are in a closed circuit state for most of the time and placed in an open circuit state only during a relatively brief measurement interval portion of the operating cycle.
Although this strategy may solve the drifting problem, it results in a limited maximum sampling frequency, and consequently also an accuracy constraint with respect to the detection of rapid changes in the flow rate. Moreover, the procedure requires a considerable amount of high-speed switching, and is therefore both expensive to implement and relatively prone to malfunction.
The object of the present invention is therefore to alleviate the problems above and thus accomplish a flow measurement solution through which an accurate fluid flow rate can be determined also when the sensor electrodes may be electrochemically affected by the fluid and its constituents.
According to one aspect of the invention, the object is achieved by the initially described flow sensor, which is characterized in that it comprises at least one direct current supply circuit adapted to feed a respective control current to each of the first and second sensor electrodes in the electric sensor arrangement. Each control currents has such sign and magnitude that a DC-voltage level at the first and second sensor electrodes relative to a reference potential is controlled towards a predetermined voltage.
An important advantage attained by this strategy is that any DC-level drift which otherwise could have occurred due to an oxidation of one or more of the sensor electrodes is prevented. At the same time, the flow measurement may be performed continuously, and/or flow rate sample values may be generated at an arbitrary frequency.
According to a preferred embodiment of this aspect of the invention, the at least one direct current supply circuit includes a differential low-pass filtering unit, an integrator unit and a controllable current generator. The differential low-pass filtering unit is adapted to generate a low-pass filtered difference signal representing a variance between the predetermined voltage and the registered voltage. The integrator unit is adapted to receive the low-pass filtered difference signal, and in response thereto, produce an adjustment signal. Finally, the controllable current generator is adapted to produce the control current in response to the adjustment signal. This design of the current supply circuit is advantageous because it accomplishes a reliable delivery of the control current with relatively simple means.
According to yet a preferred embodiment of this aspect of the invention, the at least one direct current supply circuit includes a high-input impedance buffer unit, which is adapted to receive a primary voltage signal from the electric sensor arrangement. In response thereto, the buffer unit generates a buffered voltage signal which reflects the voltage at the sensor electrodes. Naturally, an important function of the buffer unit is to prevent the control current from being fed back via the direct current supply circuit.
According to another preferred embodiment of this aspect of the invention, the conduit section includes at least one reference electrode in addition to the first and second sensor electrodes. This at least one reference electrode is adapted to produce a reference potential in the flowing fluid to which the voltage that is registered at the sensor electrodes is related. Either the at least one reference electrode applies a particular voltage, such as a ground voltage to the flowing fluid, or instead, the at least one reference electrode is used to register an externally applied voltage. In any case, the at least one reference electrode alleviates an accurate registration also of very small voltages between the sensor electrodes, which of course, is desirable feature.
According to still another preferred embodiment of this aspect of the invention, a fluid contact surface of at least one of the sensor electrodes contains platinum, for instance by being at least partly covered with platinum black. An important advantage is namely attained with this material, since it has an extremely large surface area. Thus, good electrical contact between the electrode and the fluid may be obtained via an electrode that has comparatively small dimensions.
According to another aspect of the invention, the object is achieved by the initially described ultra-filtration measuring unit, which is characterized in that it contains at least one flow sensor of the above-proposed type. Such an ultra-filtration measuring unit, for instance, enables reliable flow measurements in a dialysis apparatus where the dialysis liquid contains glucose. This is a highly desired characteristic in today's renal medicine.
According to a preferred embodiment of this aspect of the invention, the ultra-filtration measuring unit includes a measurement cell, wherein a first flow sensor is arranged with its conduit section in parallel with the conduit section of a second flow sensor, such that a common magnetic field may magnetize flowing fluid in both sensors. This design is advantageous because thereby, the circuitry for accomplishing the magnetization can be made relatively uncomplicated.
According to another aspect of the invention, the object is achieved by the initially described method for measuring a flow rate component, which is characterized by feeding at least one control current to an electric sensor arrangement for registering the voltage. The control current here has such sign and magnitude that a direct-current voltage level at the electric sensor arrangement is controlled towards a predetermined voltage, for example zero volt in relation to a reference potential.
As mentioned above, any DC-level drift which otherwise could have occurred due to an oxidation of one or more of the sensor electrodes is thereby prevented. At the same time, the flow measurement may be performed continuously, and/or flow rate sample values may be generated at an arbitrary interval. Both of these attributes are both highly desirable, particularly in medical applications.
According to a preferred embodiment of this aspect of the invention, the method includes the following steps. First, the registered voltage is compared with the predetermined voltage to produce a variance between the voltages. Then, a direct-current voltage component is extracted from this variance, which forms a basis for a control current. Finally, the control current is fed to the electric sensor arrangement, such that the direct-current voltage level at the electric sensor arrangement is controlled towards the predetermined voltage. It is generally preferable to complete these steps continuously and in parallel, such that for instance, during extraction of the DC-voltage from a particular variance, the variance between a somewhat later registered voltage and the predetermined voltage is determined, and so on.
According to yet a preferred embodiment of this aspect of the invention, the production of the control current involves the following sub-steps. First, an adjustment signal is received, which reflects the direct-current voltage component of the variance between the registered voltage and the predetermined voltage. Then, based on the adjustment signal, the control current is generated. Hence, an adaptive and straightforward regulation of the control current is accomplished.
According to another preferred embodiment of this aspect of the invention, the magnetic field has a base frequency which is different from a multiple of a typical electricity supply network frequency, such as 50 or 60 Hertz. Thereby, undesired resonance phenomena may namely be avoided.
Although the invention enhances the accuracy and reliability of flow measurements performed with respect to a fluid that risks interacting electrochemically with the sensor electrodes, the proposed solution is equally well suited for flow measurements of any non-corrosive fluids. The only requirement is that the fluid contains at least some amount of electrically charged elements.
The present invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.
A schematic view of a flow sensor according to the invention is shown in
The sensor measures a flow rate component Q along a selected direction of a fluid that flows through the conduit section 100. The selected direction (indicated with bold arrows) here coincides with longitudinal extension of the conduit section 100. The flow rate component Q is measured over the conduit section's 100 cross-section area D which, for illustrating purposes, in this example has a rectangular shape with a width w and a height h. In practice, however, any alternative cross-section shape is conceivable.
The fluid is presumed to contain electrically charged elements, such as ions. These elements are transported by the flowing fluid through the conduit section 100 at an average velocity v in the selected direction. A pair of controllable magnetizing means 110 and 111 respectively are arranged substantially opposite to each other in the conduit section 100, for example on the side walls. The magnetizing means 110 and 111 are oriented such that a magnetic axis between them is substantially perpendicular to the selected flow direction. Moreover, each magnetizing means 110 and 111 contains a conductor coil, which is electrically connected to the magnet driver 115. Thereby, by means of a magnetizing current IB, the magnet driver 115 may generate a magnetic field B across the conduit section 100 between the magnetizing means 110; 111 whose intensity depends on the magnitude of the magnetizing current IB, which is sent through the conductor coils. The polarity of this magnetic field B is determined by the sign (i.e. direction) of the magnetizing current IB.
According to Faraday's law, an electric field E is produced in the flowing fluid as a result of an interaction between the electrically charged elements in the fluid and the magnetic field B. The electric field E has its maximum component in a direction, which is perpendicular to both the direction of the (largest) flow component and the axis of the magnetic field B, and may be expressed as E=v×B, where v represents the average velocity and B represents the magnetic field.
An electric sensor arrangement is used to register the electric field E by means of a first sensor electrode 120 and a second sensor electrode 121. The sensor electrodes 120 and 121 are arranged in the conduit section 100 to be wetted by the flowing fluid therein. According to a preferred embodiment of the invention, the fluid contact surface of the sensor electrodes 120 and 121 contains platinum, and more preferably, is at least partly covered with platinum black. Furthermore, in order to enable registration of a largest possible electrical field vector, the sensor electrodes 120 and 121 are spaced apart from one another along a line which is substantially perpendicular both to the selected flow direction and the magnetic axis of the magnetic field B. The voltage sensor 125 is electrically connected both to the first sensor electrode 120 and the second sensor electrode 121, so that the voltage sensor 125 may register a voltage U between the electrodes 120 and 121.
Thus, in this example, where the sensor electrodes 120 and 121 are arranged on the floor and top walls of the conduit section 100, the distance between the sensor electrodes 120 and 121 is equal to the height h of the conduit section 100. Therefore, a voltage component VQ between the sensor electrodes 120 and 121 caused by the interaction between the electrically charged elements in the fluid and the magnetic field B may be expressed as: VQ=v·B. The relationship between the flow rate component Q and the average velocity v is: Q=v·D·B, where D denotes the cross-section area of the conduit section 100. Consequently, the voltage component VQ may be expressed as VQ=Q/D. The fact that the area D is constant renders the voltage component VQ proportional to the flow rate component Q. Moreover, it is generally preferable to measure the voltage U between the sensor electrodes 120 and 121 at time instances when the magnetic field has a particular (and substantially constant) value.
According to a preferred embodiment of the invention, the conduit section 100 includes at least one reference electrode in addition to the first and second sensor electrodes 120 and 121. The flow sensor in
Since, preferably, the polarity of the magnetic field B varies regularly, for example in response to a magnetizing current IB according to the graph in
In order to avoid the occurrence of undesired resonance phenomena, it is preferable to select the base frequency of the magnetic field B to a value being different from a multiple of a typical electrical supply network frequency (such as 50 Hz or 60 Hz). Consequently, 401 Hz constitutes one example of a suitable magnetic field frequency.
Preferably, the voltage signal U is sampled at time instances t1 and t2, when the signal has attained a stable value (e.g. corresponding to when the magnetizing current IB has reached a constant positive level IB +, shortly prior to switching over to a negative current value IB −). Due to the varying polarity of the magnetic field B, the voltage signal U also varies over time. A first relatively stable voltage level VQ + and a second relatively stable voltage level VQ − respectively (corresponding to the constant level IB + of the magnetizing current IB) may be registered at the sampling instances t1, t2 and so on. The voltage difference VQ between the voltage levels VQ + and VQ − here constitutes a measure of the flow rate Q of the fluid flowing through the conduit section 100.
According to the invention, the flow sensor also includes at least one direct current supply circuit 300, which is adapted to feed a control current Ictrl-0 to the electric sensor arrangement (i.e. each of the first and the second sensor electrodes 120 and 121) of such sign and magnitude that the DC voltage level at the sensor electrodes 120 and 121 relative a reference potential (e.g. produced by the reference electrodes 130 and 131) is controlled towards a predetermined voltage, say zero volts.
The direct current supply circuit 300 includes a differential low-pass filtering unit 320, an integrator unit 330 and a controllable current generator 340. Preferably, the direct current supply circuit 300 also includes a high-input impedance buffer unit 310, which is adapted to receive a primary voltage signal U from the electric sensor arrangement, and in response thereto, generate a buffered voltage signal U′ reflecting the voltage at the first sensor electrode 120 and the second sensor electrode 121. The differential low-pass filtering unit 320 receives the registered voltage U (either directly from the electric sensor arrangement, or via the buffer unit 310), receives a predetermined voltage DCset (typically generated on basis of values at the reference electrodes 130 and 131), and extracts there from a low-pass filtered difference signal ΔULF representing a DC-difference value between the predetermined voltage DCset and the registered voltage signal U (or U′). The integrator unit 330 is adapted to receive the low-pass filtered difference signal ΔULF, and in response thereto, produce an adjustment signal DCadj, which represents a measure of how much the DC-voltage level at the electric sensor arrangement should be adjusted in order to reach the desired voltage DCset. Thus, the differential low-pass filtering unit 320 and the integrator unit 330 together form the PI-part of a PI-regulator for the DC-voltage at the electric sensor arrangement.
The controllable current generator 340 is adapted to receive the adjustment signal DCadj, and in response thereto, produce the control current Ictrl-0, which is fed to the first sensor electrode 120 in the electric sensor arrangement. Analogous thereto, a corresponding control current Ictrl-1 is fed to the second sensor electrode 121. Thereby, the DC-voltage level at the sensor electrodes 120 and 121 is controlled towards the predetermined voltage.
The method presupposes that the fluid is magnetized by means of a magnetic field B having a periodically alternating polarity and a magnetic axis which is oriented substantially perpendicular to a selected flow direction along which the flow rate is to be measured. Moreover, a voltage is registered across the flowing fluid along a line being substantially perpendicular both to the selected flow direction and the magnetic axis.
A first step of the method 410, registers a sensor electrode voltage. Then, a step 420 extracts a DC-voltage component from the registered voltage and compares the DC-voltage component with a predetermined voltage that represents a reference value with respect to the DC-voltage level of the flow sensor. Subsequently, a step 430 generates an adjustment signal, which in turn forms a basis for producing a control current. After that, a step 440 feeds this control current to the sensor electrodes. The control current has such sign and magnitude that the direct-current voltage level at the sensor electrodes is controlled towards the predetermined voltage.
The proposed ultra-filtration measuring unit 500 includes a pump 530, a measurement cell 510, and valves 540, 550, 555 and 560. The pump 530 assists in accomplishing a desired flow Q′ of uncontaminated dialysis liquid to the dialysis apparatus Din. The valves 540, 550, 555 and 560 are used to control the ultra-filtration volume per time unit. An output flow Dout of used dialysis liquid from the dialysis apparatus is received by the unit 500 via the valve 550, and fed out for discarding Q″ via the measurement cell 510.
The measurement cell 510 contains a first flow sensor 100 a located in a first flow channel and a second flow sensor 100 b located in a second flow channel. The flow sensors 100 a and 100 b are arranged with their conduit sections in parallel with each other, such that a common magnetic field may magnetize flowing fluid in both the first flow sensor 100 a and the second flow sensor 100 b. This design is namely advantageous because thereby, the circuitry for accomplishing the magnetization can be made relatively uncomplicated. The measurement cell 510 measures the flow rate of dialysis liquid into the dialysis apparatus Din via the first flow channel, and a corresponding flow rate Dout from the dialysis apparatus via the second flow channel. Thus, by measurements performed in the measurement cell 510, the flow rates Din(Q′) and Dout (Q″) can be controlled to obtain a desired ultra-filtration volume per time unit.
In order to prevent (or at least minimize) calculation errors in the flow measurements, the measuring unit 500 performs self-calibration, by means of a so-called taration procedure, at regular intervals (typically once every 30 minutes during the dialysis treatment).
The taration procedure includes one phase in which a zero-flow is measured. Here, the valve 540 is open and the valves 550, 555 and 560 are closed, so that the flow Q′ of uncontaminated dialysis liquid is fed out directly for discarding Q″ without passing the measurement cell 510 or the dialysis apparatus.
In another phase of the taration procedure, a differential flow between the two flow channels is measured and set to zero. Here, the valve 555 is open while the valves 540, 550 and 560 are closed, so that the flow Q′ of uncontaminated dialysis liquid passes the measurement cell 510 via both the first and the second flow channels, and is then fed out for discarding Q″ (i.e. without passing the dialysis apparatus). Preferably, in this phase, the flow Q′ is set to a standardized and well-defined value, say 500 ml/minute.
Based on the above zero-flow- and differential-flow-measurements any adjustments necessary to the flow rate calculations performed on the values delivered by the measurement cell 510 can be made, so that systematic future calculation errors are avoided.
The term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, integers, steps or components or groups thereof.
The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US4612806 *||Nov 2, 1984||Sep 23, 1986||Feller Murray F||Orbital ball flow detection apparatus|
|US4616509 *||Jan 15, 1985||Oct 14, 1986||Feller Murray F||Flow detectors|
|US5392657 *||Feb 22, 1993||Feb 28, 1995||Onicon Incorporated||Flow sensor having high impedance circuit with capacitive sensing electrode|
|US6431011||Mar 28, 2001||Aug 13, 2002||Murray F. Feller||Magnetic flow sensor and method|
|US20020050175||Mar 28, 2001||May 2, 2002||Feller Murray F.||Magnetic flow sensor and method|
|US20020145417||Apr 4, 2002||Oct 10, 2002||Helmut Brockhaus||Magnetoinductive flowmeter and magnetoinductuve flow-measuring process|
|US20030005777||May 31, 2002||Jan 9, 2003||Thomas Budmiger||Method of operating an electromagnetic flowmeter|
|JPS60237321A||Title not available|
|International Classification||G01F1/60, G01F1/58, G01F1/56|
|Cooperative Classification||A61M1/341, A61M1/1647, G01F1/588, A61M1/16, G01F1/60|
|European Classification||G01F1/58D, A61M1/16, G01F1/60|
|Aug 4, 2005||AS||Assignment|
Owner name: GAMBRO LUNDIA AB, SWEDEN
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:HALLSTADIUS, HANS;OHLSSON, GORAN;GUSTAFSON, SVEN;AND OTHERS;REEL/FRAME:017291/0605;SIGNING DATES FROM 20050415 TO 20050426
|Oct 16, 2007||CC||Certificate of correction|
|May 21, 2009||AS||Assignment|
Owner name: CITICORP TRUSTEE COMPANY LIMITED,UNITED KINGDOM
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Effective date: 20090331
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